In recent years, an excimer laser apparatus and a molecular fluorine laser apparatus are taken into consideration and adopted as a lithography light source. In such a case, the following two points are required in order to improve the throughput of an exposure device and to conduct ultrafine fabrication uniformly.
A first point is to provide a high power laser beam. To provide high power, there is a method of increasing a pulse energy per pulse of a laser pulse which is output from a laser apparatus. And, if the pulse energy per pulse is low, a shortage of energy can be compensated by a method of increasing a repetition frequency.
A second point is to provide a very narrow-band spectrum. To provide a very narrow-band spectrum, there are, for example, a method of providing a high resolution line narrow module (hereinafter referred to as the “LNM”) which is configured of a prism and a grating, and a method of providing a long pulsed laser as described in the following Patent Literature 1 given below.
But, the provision of the LNM with the high resolution ability and the long pulse degrades the pulse energy because an optical loss is generally increased. In other words, the very narrow banding of the spectrum and the increase of the pulse energy are incompatible with each other. When it is assumed that the cost is to be reduced, the increase of a repetition frequency, for example, a repetition frequency of exceeding 4 kHz is hard to achieve technically. Therefore, the realization of an excimer laser apparatus or a molecular fluorine laser apparatus having only one chamber provided with high power by increasing a repetition frequency while keeping a very narrow-band spectrum has limitations.
To satisfy both the above-described two points, a two stage laser having an optical oscillation stage and an optical amplification stage is proposed by, for example, the following Patent Literature 2 and the like. Both the optical oscillation stage and the optical amplification stage have a chamber in which laser gas is sealed and a pair of opposed electrodes are disposed. When a discharge occurs between the electrodes in the oscillating chamber, the laser gas is excited to transit to an excited state, and light is produced at the time of transition from the excited state to a ground state. When a light energy is amplified to some extent, a laser beam (seed light) is output from the oscillating laser. The output seed light is injected into the amplifying chamber. When a discharge is generated between the electrodes in the amplifying chamber, the energy of the injected seed light is amplified and output.
The two stage laser is roughly divided into two types. One of them is a type in that an oscillating laser apparatus is disposed in an optical oscillation stage, and an amplifier not having a light resonator is disposed in an optical amplification stage and called as an MOPA system (Master Oscillator Power Amplifier). The other is a type in that an oscillating laser apparatus is disposed in an optical oscillation stage and an amplifying laser apparatus having a light resonator is disposed in an optical amplification stage and called as an MOPO system (Master Oscillator Power Oscillator).
The optical oscillation stage is provided with an LNM to realize a very narrow-band spectrum. Meanwhile, in the optical amplification stage, only the energy is amplified while keeping a very narrow-band spectrum of the seed light which is output from the optical oscillation stage and injected into the amplifying chamber. According to this two-stage laser, the optical amplification stage does not include an element which becomes an optical loss of the LNM and the like, so that laser oscillation efficiency is very high. Therefore, it becomes possible to obtain a desired very narrow-band spectrum and laser output. The desired laser output is the product of a pulse energy per pulse and repetition frequency. For example, performance required for a next-generation ArF excimer laser includes a spectral FWHM (Full Width at Half Maximum) of 0.25 pm or less and laser output of 40 W or more when operating at a repetition frequency of 4 kHz.
To reduce damage to the optical system provided for the exposure device, it is desirable that a light pulse waveform is low peak power. Accordingly, long pulsization is demanded. And, high repetition is demanded because high power is required.
Where a laser apparatus has a single chamber only, it is controlled to obtain a prescribed energy by predictive determination (feedback) of a charging voltage HV for a next pulse in view of a relationship between a supply voltage (charging voltage) HV of a pulse earlier by one to several pulses or one to several bursts and a pulse energy. Besides, it is controlled to raise the charging voltage HV in order to compensate a medium-term output drop, and to increase a laser gas pressure in order to compensate the long-term output drop. To control the pulse energy integrated value into a target range, a charging voltage HV of the next pulse is determined according to a difference between the cumulative energy up to the previous pulse and the target cumulative energy so that a prescribed integrated value can be obtained.
Patent Literature 1: Japanese Patent Laid-open Publication No. 2001-156367
Patent Literature 2: Japanese Patent Laid-open Publication No. 2001-24265 (FIG. 1)